The present invention relates to a process for pulse periodic reverse (xe2x80x9cPPRxe2x80x9d) electroplating. More particularly, this process involves varying the anodic to cathodic current density ratio in order to improve the surface appearance, grain structure and levelling of the deposit, while maintaining uniform metal distribution and appearance on the substrate when plating at high current densities.
In conventional electroplating, an aqueous electrolytic solution can be prepared by dissolving a metal salt, such as copper sulfate, in sulfuric acid and water. If desired or necessary, additional agents, such as surfactants, brighteners, antioxidants and the like can be included in the solution to impart certain performance characteristics to the solution or deposit. The electrodes of an external circuit are then immersed into this electrolytic solution, and a DC current is applied across the electrodes. This causes an electrochemical reaction or reduction, resulting in the deposition of metal onto the cathode from the metal ions in the electrolytic solution. The current density profile and primary current distribution across the cathode varies according to the geometric path or distance between the anode and cathode, leading to deposit thickness variations according to the shape and location of the cathode relative to the anode. This effect is most apparent when high applied average current densities are used. Therefore, in order to obtain the best uniformity of metal distribution, low applied average current densities are used.
Alternatively, by using PPR current instead of DC current, uniform metal deposits can be produced at higher current densities. This technique is especially useful for electrolytic copper plating on high aspect printed circuit boards, which are relatively thick boards with small hole diameters. These substrates present particular problems because of their surface geometry, which affects current distribution, and results in measurable differences in current density between the surfaces of the board and the through-hole. This current density difference causes uneven metal deposition, with thicker coatings produced on surfaces with higher current densities. Generally, the edges of the board and isolated surface circuitry experience the higher current density and resultant thicker deposit compared to the center surfaces of the board or the inner surfaces of the hole. The additional thickness in these areas can present significant problems in subsequent processing and assembly operations. A non-uniform surface profile leads to increased soldermask being required to meet minimum thickness requirements for good coverage. The lack of circuit planarity and excess thickness at the hole entry can interfere with the proper location of components during assembly, while the methods used to reduce this excess thickness lead to excessive processing times and a loss of production.
With PPR current, also known as pulsed periodic reverse current, it is possible to produce uniform coatings with an even thickness on both the surface of the board and in the through-hole. A pulsed periodic reverse current is created by alternating the current modulation between forward and reverse cycles. Specifically, this is accomplished by inverting the current from cathodic to anodic mode, which disrupts the otherwise constant direct current polarization effects. The degree of disruption occurs according to the primary current distribution, with more in the high current density areas than in the low current density areas, thus providing a normalization of deposition rates across complex geometries at higher applied average current densities. Moreover, by maintaining thickness uniformity at higher applied average current densities, the overall deposition rate is increased and processing times reduced thereby yielding higher production output.
Although the use of pulse periodic reverse current results in uniform coating thicknesses at high current densities, the surface appearance of the resulting deposit will range from a matte to a semi-bright finish, relative to the hole wall thus producing a non-uniform deposit appearance between high (surface) and low (hole) current densities. On the other hand, if DC current is applied, uniformly bright deposits are produced, throughout the current density range, but low current densities must be used in order to preserve coating thickness uniformity. Thus, neither method provides optimum thickness distribution with uniform metal deposit appearance at high current densities.
Electroplating methods are used to deposit metal onto substrates for various purposes, such as providing a decorative or protective coating, improving solderability, reducing contact resistance, increasing surface conductivity or reflectivity, and improving hardness and wear resistance. With these wide range of useful purposes in mind, it would be very beneficial to develop an electroplating technique which produces a metal coating with a uniform finish, while maintaining high current density throwing power at high applied current densities.
The invention relates to a method of electrodepositing metal onto a substrate, by applying a pulsed periodic reverse current across the electrodes of a plating cell utilizing a peak reverse current density and peak forward current density, and varying the ratio of peak reverse current density to peak forward current density in periodic cycles to provide a metal deposit of uniform appearance, fine grain structure and uniform thickness onto the substrate. A preferred, convenient way to vary this ratio is by holding the peak forward current constant while varying the peak reverse current density.
Advantageously, the ratio of peak reverse current density to peak forward current density has at least a first value in a first time period, and a second, different value in a second time period, and the uniform thickness and appearance of the metal deposit is achieved by varying the ratio between the first and second values. Preferably, the ratio of peak reverse current density to peak forward current density has a first value in a first time period, a second value in a second time period, and a third value in a third time period, and the uniform thickness and appearance of the metal deposit is achieved by varying the ratio sequentially between the first, second and third values. A more preferred method is to vary the ratio sequentially between the first, second, third, second and first values.
The electrolytic solution that is used typically contains agents that are stable in the solution and produce desired improvement in the metal deposit. When bright copper deposits are desired, the electrolytic solution is an aqueous acid copper electrolyte, produced by combining sulfuric acid, copper sulfate, and chloride anions. One prominent use for such bright copper deposits is upon a printed circuit board substrate having a plurality of through holes therein. The present process allows one to provide a copper deposit of uniform appearance, fine grained structure and uniform thickness both on the surface of the circuit board and in the through holes.
Another embodiment of the invention relates to a method of enhancing the properties of an electrodeposit which comprises providing the electrodeposit upon a substrate by a pulse periodic reverse electroplating process while varying the anode/cathode current density ratio to provide a metal deposit of uniform appearance, fine grain structure and uniform thickness. When the substrate has uneven surfaces or apertures therein, this method enables one to provide a metal deposit of uniform appearance, fine grain structure and uniform thickness both on the surface of the substrate and in the through holes.